U.S. Pat. No. 4,657,394 (Halioua), issued on Apr. 14, 1987, discloses an apparatus and method for measuring three-dimensional surface contours, whereby phase measurements are taken of deformed, two-dimensional grating images and these phase measurements are used to perform an improved optical profilometry. Individual line profiles are obtained at a series of rotational increments of a body. A full 300.degree. surface profile or a portion thereof can be generated by illuminating the three-dimensional image from a grating projector combined with a phase shifter and the reflected light is received by a linear array camera which provides output signals to an analog-to-digital converter and to a central processing unit which in turn provides respective output signals to a display. Halioua relies on the diffusion of a laser beam by the curved surface to be measured. A very precise focussing is required especially where a sharp edge is to be inspected so that the edge is positioned exactly in the focal plane. Such exact positioning requires a substantial effort and expense of position adjustment devices. No use is made of the optical fact that in a specular reflection the incidence angle equals the reflection angle.
U.S. Pat. No. 4,709,156 (Murphy et al.), issued on Nov. 24, 1987, discloses a method and apparatus for inspecting a curved three-dimensional surface, for example, in the form of propulsion plant blades. Murphy et al. scan the surface with a laser beam and the beam reflected by the surface is received by a detector array or linear detector for ascertaining the curvature profile. The distance between locations at which the reflected light beams are focused on the linear detector corresponds to the distance at which points on the surface of the article are offset due to the curvature of the surface being scanned. This type of arrangement has the disadvantage that it cannot ascertain the quality of curvature profiles of sharp edges having curvature radii on the order of the cross-section of the illuminating laser beam. The Murphy et al. device is capable of scanning only slightly curved surfaces, for example, the surface of a blade or vane, but not the sharply curved edges of such a vane or blade, especially the trailing edge.
European Patent Publication 0,318,249 A2 (Penney et al.), published May 31, 1989, discloses a ranging system which provides a high speed read-out of a triangulation performed with the aid of a coded aperture light detector which provides a direct digital representation of the range or elevation of a surface spaced from the ranging device. A light spot reflected by the scanned surface is optically spread into a line segment so that it can be shared among a number of light detection channels. The line of light falls on a coded aperture in front of a segmented bundle of fiber optic conductors and the light transmitted by each light conductor is led to a separate photomultiplier or solid state detector. Every coded channel is constructed to give one bit of the digital address of the range position and a reference light value is obtained from another channel. Background reflections and secondary reflections are filtered out by focusing light scattered from the surface to a spot and passing it through a slit aperture oriented in the plane of triangulation before being spread to a line segment. Such a device is primarily used in a three-dimensional camera.
German Patent Publication (DE-OS) 2,412,359 (Baerwinkel et al.), published on Sep. 25, 1975, discloses a method and apparatus for the contactless measuring or surveying of object contours. The object is illuminated with coherent light from two point-shaped light sources to form a three-dimensional interference field which in turn produces on the object being scanned an optical interference strip pattern that in turn is imaged onto an image plane by a central projection. An interference strip pattern is produced on a reference plane having at least three points with a known position. The data of the optical arrangement of the light sources, the imaging optic, and the image plane, are ascertained from the interference strip pattern from the imaging of said pattern and from the image of the known points. A non-expanded laser beam extending in the direction of the central vertical on a line connecting the two light sources marks the interference of null order. A computer calculates the position in space of the corresponding object points and thus the object contour on the basis of the coordinate of the image points and on the basis of the corresponding interference orders. Here again, very sharp contours cannot be imaged.
German Patent Publication (DE-OS) 2,620,330 (Hockley), published on Nov. 18, 1976, discloses a method and apparatus for determining a surface configuration. The Hockley apparatus includes an illuminating device which illuminates the surface to be measured or ascertained with an image. The image comprises straight parallel lines or strips that are light and dark so that a corresponding pattern is formed on the surface of the object. The configuration of the object surface distorts the pattern. A scanner and transducer scans the distorted pattern in order to follow a path along the surface and to produce a respective electrical output signal which indicates the variations in the surface illumination along the scanned paths, whereby these illumination changes are caused by the distortion of the strip pattern. Here again, there is room for improvement regarding the inspecting of sharp edges.
German Patent Publication (DE-OS) 3,817,561 (Berger et al.), published on Nov. 30, 1989, discloses an apparatus for projecting an object grid pattern used for making topographic measurements on an object to be tested. The Berger et al. apparatus makes use of the "projection-Moire-method". For this purpose a laser projects its light onto a two-beam interferometer having two mirrors and a beam divider. At least one of the mirrors is a mirror arrangement for producing curved wave fronts. One of the mirrors is a fixed spherical mirror and the other is an adjustable spherical mirror. A transversal displacement of the adjustable mirror makes it possible to adjust the grid constant of the object grid pattern. An axial displacement of the adjustable mirror provides for an adjustment of the phase position of the object grid pattern.
U.S. Pat. No. 5,003,187 (Zumbrunn et al.), issued on Mar. 26, 1991, discloses a method and apparatus for surveying the surface of an object by projecting fringe patterns whereby gratings with linear line patterns are used. The line patterns have different periods and are successively projected onto the surface being surveyed. A television camera detects images of the line patterns as deformed by the surface being surveyed. The deformed images or their phase positions relative to a reference plane are stored and evaluated by an electronic circuit. The distortions of the line patterns are computed to provide information of absolute heights of points relative to the reference plane. Zumbrunn et al. direct a beam of light having a periodically varying intensity pattern over the beam cross-section, at the surface to be inspected. Defined phase positions yield residual phase values from which the heights of points on the inspected surface relative to the reference plane are Calculated. Zumbrunn et al. require diffused reflection and do not use specular or mirror reflection. Besides, a single measurement requires a plurality of line pattern projections each having a different period. These features of Zumbrunn et al. can be realized only with a substantial effort and expense for optics, etc.
U.S. Pat. No. 5,067,817 (Glenn), issued on Nov. 26, 1991, discloses a method and apparatus for measuring of the curvature and profile of a reflective test surface. The slope is measured by simultaneously evaluating two closely spaced points on the test surface by directing two parallel, nearly collimated light beams, which are slightly displaced relative to each other and of opposite linear polarization onto the surface to be tested. The laser beam used must have a beam cross-section substantially smaller than the surface area to be measured. Glenn also relies on diffused reflection and his system components, especially the optics, are rather involved.